It is known that copolymerization of ethylene with a minor proportion of an alpha-olefin with a Ziegler catalyst gives an ethylene copolymer having the same density as high-pressure polyethylene. Generally, it is advantageous to employ a high-temperature solution-polymerization process in which the polymerization is carried out by using a hydrocarbon solvent at a temperature above the melting point of the resulting copolymer because the polymerization operation is easy. However, attempts to obtain a polymer having a sufficiently high molecular weight result in higher viscosities of the polymer solution, and the polymer concentration in the polymer solution should be lowered. Accordingly, the process has the defect that the productivity of the copolymer production per polymerization vessel necessarily becomes low.
On the other hand, when it is desired to obtain the aforesaid low-density ethylene copolymer using known catalysts by the slurry polymerization method frequently used in the production of high-density polyethylene, the copolymer is frequently liable to dissolve or swell in the polymer solution, resulting in the rise of the viscosity of the polymer solution, the adhesion of the polymer to the polymerization vessel walls and the decrease of the bulk density of the polymer. Accordingly, this method has the defect that the slurry concentration cannot be increased, and it cannot be operated continuously for a long period of time. Furthermore, the resulting copolymer becomes sticky, and its quality is affected.
It has been found that the aforesaid technical defects or troubles are especially serious in the production of low to medium density ethylene copolymers from a major proportion of ethylene and a minor proportion of an alpha-olefin having 3 to 10 carbon atoms.
Investigations of the present inventors have shown that the aforesaid technical defects or troubles are especially great in the production of ethylene copolymers having a density of 0.910 to 0.945 g/cm.sup.3 of ethylene as a main component and a minor amount of an alpha-olefin having 3 to 10 carbon atoms as a comonomer which can give film products having excellent transparency and heat-sealing property.
Investigations of the present inventors have shown that in the production of the above ethylene copolymers by the slurry polymerization method, the resulting solid polymer is liable to swell in the reaction solvent and to assume an irregular shape far from a desirable shape such as a sphere or the like, and therefore if the concentration of the polymer in the polymerization system is increased, the polymerization system assumes a porridge-like slurry which makes it difficult to perform a polymerization operation permitting uniform polymerization It has further been found that it is impossible to avoid the formation of a substantial amount of an ethylene copolymer easily soluble in hydrocarbon solvents, which is indicated, for example, as the proportion of the hexane-soluble portion in Examples and Comparative Examples given hereinafter, and consequently the viscosity of the copolymer solution increases unduly. These technical defects attributed to the poor slurry properties during the polymerization have been found to be difficult to circumvent.
Thus, because of the poor slurry properties during slurry polymerization for the production of the specific ethylene, the slurry concentration of the copolymerization system cannot be increased, and therefore, the desired ethylene copolymer cannot be produced with high productivity. Furthermore, increasing the slurry concentration of the copolymerization system causes various troubles such as an abrupt reduction in stirring efficiency, blockage of pipes for conveying the slurry, the reduced efficiency of separating the copolymer from the reaction solvent in a decanter, and the increased load of energy required to dry the separated copolymer.
When a vapor-phase polymerization method is used instead of the slurry polymerization method, it is likewise impossible to avoid the formation of a substantial amount of an ethylene copolymer easily soluble in hydrocarbon solvents. Moreover, since the ethylene polymer shows undesirable tackiness in the vapor phase copolymerization system, the copolymer particles are agglomerated in the vapor-phase polymerization reaction zone, and a stable fluidized bed becomes difficult to form. Furthermore, it is difficult to circumvent blockage of that part of the polymerization vessel which is near a port for withdrawing the resulting ethylene copolymer.
No proposal has yet been known of using the titanium catalyst component specified in the process of this invention with a view to overcoming the aforesaid technical defects or troubles which constitute an especially important technical problem in the production of an ethylene copolymer having a density of 0.910 to 0.945 g/cm.sup.3 and composed of a major proportion of ethylene and a minor proportion of alpha-olefin having 3 to 10 carbon atoms.
Some prior proposals are known of polymerizing or copolymerizing olefins in the presence of a catalyst composed of a titanium catalyst component, which is the reaction product of a hydrocarbon solvent-insoluble magnesium/aluminum solid complex derived from a liquid magnesium compound and an compound, and an organoaluminum compound catalyst component. However, no proposal has been known of overcoming the aforesaid technical defects or troubles in the production of the above-specified ethylene copolymer.
For example, Japanese Laid-Open Patent Publication No. 11908/1981 (corresponding to European Laid-Open Patent Publication No. 22675) proposes a process for polymerizing or copolymerizing olefins in the presence of a catalyst which can embrace the aforesaid catalyst composed of the titanium catalyst component and the organoaluminum compound catalyst component. This proposal, however, does not at all refer to the presence of the aforesaid technical problem in the production of the specific ethylene copolymer and its solution. All of the working examples given in this patent document are directed to the polymerization of propylene. Naturally, therefore, this patent publication neither describes nor suggests the parametric combination of the requirements (i) to (iv) of this invention to be described in detail hereinafter. It neither gives a specific disclosure of any one of these requirements. In particular, as regards the requirement (iii), the above patent document states that the Ti/Mg (atomic ratio) is at least 1, usually about 5 to about 200, especially about 10 to about 100. As will be shown by Comparative Example 5, the use of the Ti/Mg atomic ratio which does not meet the requirement (iii) cannot achieve the unexpected and excellent improving effect by the process of this invention.
Japanese Laid-Open Patent Publication No. 189206/1983 (corresponding to European Laid-Open Patent Publication No. 93494) also proposes a process which comprises polymerizing or copolymerizing olefins in the presence of a catalyst which can embrace the catalyst composed of a titanium catalyst component, which is the reaction product of a hydrocarbon solvent-insoluble magnesium/aluminum solid complex derived from a liquid magnesium compound and an organoaluminum compound, and an organoaluminum compound catalyst component. This patent document neither refers to the presence of the aforesaid technical problem in the production of the aforesaid specific ethylene copolymer and its solution. All of the working examples in this patent document are directed to homopolymerization of ethylene/propylene or butene-1. In this proposal, too, it is natural that no specific disclosure is made of the parametric combination of the requirements (i) to (iv) of the present invention or of any of these requirements individually.
It is an object of this invention therefore to provide a process for producing an ethylene copolymer having a low to medium density and composed of a major proportion of ethylene and a minor proportion of an alpha-olefin having 3 to 10 carbon atoms, which can overcome the aforesaid technical problem in the production of low to medium density ethylene copolymers.